Rhodococcus equi

Introduction

Rhodococcus equi is a gram-positive aerobic, intracellular, saprophyte found worldwide in the soil and in the intestinal tract of animals. It replicates at warm temperatures in soils contaminated by faeces of herbivores and is an opportunistic respiratory pathogen of foals under 6 months causing suppurative bronchoneumonia. 

Pastures can become heavily contaminated leading to outbreaks of the disease. The organism can be present in the faeces of adult horses and also healthy foals, without causing clincal disease. Transmission can be via inhalation of dust contaminated with R. equi.

Clinical Signs

Bronchoneumonia and lung abscessation in foals less than 4 months old. Acute disease in 1 month-old foals with signs such as acute fever, anorexia, bronchopneumonia. In older foals, 2- 4 months the disease is more insidious with signs such as coughing, dyspnoea, weight loss, exercise intolerance, loud, moist rattlles on lung auscultation. There is occasionally diarrhoea seen, due to granulomatous ulcerative enterocolitis, mesenteric lymphadenitis and peritonitis in some foals following ingestion of contaminated sputum.

Foals over 6 months refractory to pulmonary infection, but superficial abscesses may occur in in horses over 6 months.

Diagnosis

History of the disease on the farm, age of foals and clinical signs are used to make a presumptive diagnosis. Auscultation and radiography of thorax confirms pulmonary disease.

Samples of   tracheal aspirates and pus from lesions need to be taken and cultured aerobically on blood and MacConkey agar for 24-48 hours. Characteristic salmon-pink mucoid colonies grow, due to capsule and pigment production. 

Treatment and Control

Oral rifampin and erythromycin for 4-10 weeks is the treatment of choice for this disease. Rehydration therapy if diarrhoea is invloved may be needed. Bronchodilators and expectorantscan be used supportively if pneumonia is severe.

Control measures include preventing build-up of bacteria in environment by removing manure from pastures regularly and rotating foals and mares onto clean pastures regularly. Dusty conditions in paddocks should be avoided and poo- picking should be performed regularlyto keep pastures clean. In sickly looking foals, hyperimmune serum from the dam may be given to the foal in the first month of life. 

Pyogranulomatous lesions due to Rhodococcus equi (Image sourced from Bristol Biomed Image Archive with permission)

References

Brown, C.M, Bertone, J.J. (2002) The 5-Minute Veterinary Consult- Equine', Lippincott, Williams & Wilkins
Reed, S.M, Bayly, W.M. and Sellon, D.C (2010) Equine Internal Medicine (Third Edition), Saunders.
Reed, S.M, Bayly, W.M, Sellon, D.C. (2004) Equine Internal Medicine (Second Edition) Saunders.
Robinson, N.E., Sprayberry, K.A. (2009) Current Therapy in Equine Medicine (Sixth Edition) Saunders Elsevier
Rose, R. J. and Hodgson, D. R. (2000) Manual of Equine Practice (Second Edition) Sauders.

Literature Search

Use these links to find recent scientific publications via CAB Abstracts (log in required unless accessing from a subscribing organisation).


Rhodococcus equi infection in horses: an overview. Rajesh Agrawal; Nishi Pande; Rajesh Katoch; Anish Yadav; Ajitpal Singh; Veterinary Practitioner, c/o Dr. A. K. Gahlot, Bikaner, India, Veterinary Practitioner, 2009, 10, 2, pp 187-188, 16 ref. - Full test article

Review of the epidemiology and ecology of Rhodococcus equi. Muscatello, G.; Lowe, J. M.; Flash, M. L.; McBride, K. L.; Browning, G. F.; Gilkerson, J. R.; Green, E. M. ; American Association of Equine Practitioners (AAEP), Lexington, USA, Proceedings of the 53rd Annual Convention of the American Association of Equine Practitioners, Orlando, Florida, USA, 1-5 December, 2007, 2007, pp 214-217, 27 ref. - Full test article